It is the conventional practice to use a zinc sulfate electro-galvanizing solution mainly comprising zinc sulfate (hereinafter referred to as the "sulfate galvanizing solution") as an acidic electro-galvanizing solution. The sulfate galvanizing solution has the following defects as compared with a zinc chloride electro-galvanizing solution comprising zinc chloride and other chlorides (hereinafter referred to as the "chloride galvanizing solution"):
(1) a very low electric conductivity; PA1 (2) requiring an excessive voltage as the bath voltage in order to obtain a prescribed electro-galvanizing current density, thus resulting in a high electric power cost; PA1 (3) a low allowable electro-galvanizing current density making it difficult to electro-galvanize a steel sheet at a high speed. PA1 zinc chloride (ZnCl.sub.2) from 100 to 300 g/(, and PA1 at least one substance selected from the group consisting of ammonium chloride (NH.sub.4 Cl), potassium chloride (KCL), sodium chloride (NaCl), aluminum chloride (AlCl.sub.3), barium chloride (BaCl.sub.2), calcium chloride (CaCl.sub.2), magnesium chloride (MgCl.sub.2), managanese chloride (MnCl.sub.2) and iron chloride (FeCl.sub.2, FeCl.sub.3): from 100 to 400 g/l; PA1 (hereinafter referred to as the "Prior Art") PA1 zinc chloride: from 100 to 400 g/l, PA1 at least one substance selected from the group consisting of ammonium chloride and potassium chloride: from 100 to 400 g/l, and PA1 at least one substance selected from the group consisting of saturated carboxylic acid, sodium salt thereof and potassium salt thereof: from 1 to 70 g/l.
As compared with the sulfate galvanizing solution, on the other hand, the chloride galvanizing solution is very corrosive and easily causes corrosion of a galvanizing tank or a piping of an electro-galvanizing apparatus. Adoption of the chloride galvanizing solution has therefore been hesitated. Recently, however, there has been made a remarkable progress in the development of materials having an excellent corrosion resistance to acids, and as a result, adoption of the chloride galvanizing solution is being reconsidered.
However, application of continuous electro-galvanizing to a steel sheet at a high speed with the use of the chloride galvanizing solution involves the following problems.
When electro-galvanizing a steel sheet using the chloride galvanizing solution, a zinc electrode is employed as a soluble anode. A scaly black sludge (hereinafter referred to as the "anode sludge") in a very large amount is produced on the surface of the zinc electrode in the chloride galvanizing solution. More particularly, the chloride galvanizing solution has an electric conductivity for higher than that of the sulfate galvanizing solution and contains much chlorine ions, so that the chloride galvanizing solution has a corrosivity far higher than that of the sulfate galvanizing solution. Therefore, when continuously electro-galvanizing a steel sheet using the chloride galvanizing solution, Fe.sup.2+ ions in a far larger amount than in the sulfate galvanizing solution are dissolved from the surface of the steel sheet into the chloride galvanizing solution, and are accumulated therein. Since a local battery is formed on the surface of the zinc electrode even when electricity is on, these Fe.sup.2+ ions are easily replaced into metallic iron which precipitates on the surface of the zinc electrode. Because of the high dissolving rate and the high electric conductivity of zinc, production and separation of a scaly metallic iron onto and from the surface of the zinc electrode are repeated at a frequency far higher than in the sulfate galvanizing solution, and the anode sludge is thus produced in a very large amount.
The anode sludge separated from the surface of the zinc electrode floats in the chloride galvanizing solution. The anode sludge thus floating in the chloride galvanizing solution adheres onto the surface of the electro-galvanized steel sheet to stain the surface thereof or adheres onto the surfaces of the rolls of the electro-galvanizing apparatus to form dent defects on the surface of the electro-galvanized steel sheet. In addition, the anode sludge floating in the chloride galvanizing solution adheres onto the pump and the piping of the electro-galvanizing apparatus to cause clogging or is dissolved again into the chloride galvanizing solution as impurity ions. The anode sludge thus impairs smooth operation of electro-galvanizing and degrades quality of the electro-galvanized steel sheet.
As a chloride galvanizing solution for improving the exterior appearance of an electro-galvanized steel sheet, Japanese Patent Provisional Publication No. 57-89,493 dated June 3, 1982 discloses an acidic electro-galvanizing solution comprising:
pH value of said acidic electro-galvanizing solution being adjusted within the range of from 1.0 and 3.5.
However, the Prior Art cannot solve the problems mentioned above regarding production of the anode sludge. More specifically, the chloride galvanizing solution as the acidic electro-galvanizing solution according to the Prior Art does not contain a constituent having the function of inhibiting production of the anode sludge. Furthermore, this chloride galvanizing solution, having a low pH value within the range of from 1.0 to 3.5, is very highly corrosive. As a result, when electro-galvanizing a steel sheet with the use of the chloride galvanizing solution according to the Prior Art, not only Fe.sup.2+ ions are dissolved from the surface of the steel sheet into the chloride galvanizing solution, but also metallic ions are dissolved from the metal parts of the electro-galvanizing apparatus which are in contact with the chloride galvanizing solution, and a local battery tends to be formed on the surface of the zinc electrode, so that the amount of the produced anode sludge seriously increases. In addition, as the chloride galvanizing solution has a low pH value as described above, the produced anode sludge is liable to be easily dissolved again into the chloride galvanizing solution. It is therefore inevitable that a large amount of ions of impurities are accumulated in the chloride galvanizing solution.
Under such circumstances, there is a strong demand for the development of an acidic electro-galvanizing solution which permits manufacture of an electro-galvanized steel sheet having a satisfactory quality by largely inhibiting production of the anode sludge, which is the most detrimental defect of the conventional chloride galvanizing solution, without impairing the low electric power cost and the high productivity, which are the favorable merits of the chloride galvanizing solution, but such an electro-galvanizing solution has not as yet been proposed.